PETROLOGICAL ABSTRACTS AND REVIEWS e/a 
edges that as yet “‘little or no attempt has been made to define more 
precisely what constitutes the characteristics of any so-called petro- 
graphical province.” ‘“‘ Nothing approaching completeness of definition, 
either as to composition of the rocks, or extent and limit of the region 
of occurrence, has ever been attempted.”’ Iddings does not believe that 
the major provinces or regions: of the earth, thought by Becke, Harker, 
and others to be characterized by the ‘“‘Atlantic and Pacific” or the 
‘alkaline and calcic”’ branches, are entitled to recognition. 
From Iddings’ standpoint the problem before petrologists. today, 
respecting geographic distribution, “is the investigation and exact 
definition of the districts and regions of igneous rocks in all parts of the 
world, with the purpose of obtaining the data with which to form definite 
conceptions of what have been termed petrographical provinces.”’ Exist- 
ing data make it clear to him “that there are many kinds of such groups 
of igneous eruptions and not two strongly contrasted series; that they 
blend into one another in composition; that the delineation of the 
regions, or provinces, may be pronounced in some instances and ill-defined 
in others.”’ 
In view of Iddings’ opinion concerning the very incomplete data of 
igneous rocks themselves, their relations and distribution, it is natural to 
find him expressing the belief that major problems such as ‘‘the relation 
of the composition of igneous rocks of different parts of the earth to its 
isostacy”; “the relation between the kinds of magma erupted in a 
particular region and the dynamical events within the region’”’; and the 
question of original homogeneity or heterogeneity in the earth, must 
wait for their reasonable solution on the accumulation of a great amount 
of exact data, by petrographer, chemist, geophysicist, and geologist. 
The article by the present writer on “the natural classification of 
igneous rocks,”’ which antedates the other discussions here reviewed may 
be briefly referred to. In the first place, petrographers and petrologists 
are reminded that a classification of rocks may be called natural only 
when the factors used in its construction are really the facts or relations 
of nature. It is pointed out that the generalizations used in many 
attempted classifications were so erroneous or inaccurate as to make the 
result peculiarly unnatural. The special object of the discussion is a 
defense of the quantitative classification of igneous rocks from the 
criticism urged most forcibly by the advocates of natural classification 
that the quantitative method is unnatural and arbitrary. 
The authors of the quantitative system were moved to its formulation 
